Not a biologist or neurologist, but my hypothesis for this is ease of blood flow to the brain while horizontal.
Occam's Razor: The (simplest) explanation with the fewest assumptions tends to be the correct one.
It's almost always that simple.
Also, when you're travelling or on the train, and your wonder sense opens, you are relaxed. Less constriction to the arteries and blood vessels. As with lying horizontal, increased blood flow activates the brain.